diff -up a/build/moz.configure/lto-pgo.configure.pgo b/build/moz.configure/lto-pgo.configure

--- a/build/moz.configure/lto-pgo.configure.pgo	2020-09-30 19:41:10.000000000 +0200

+++ b/build/moz.configure/lto-pgo.configure	2020-10-01 14:48:15.459225300 +0200

@@ -69,12 +69,14 @@ set_config('PGO_PROFILE_PATH', pgo_profi

 @imports(_from='__builtin__', _import='min')

 def pgo_flags(compiler, profdata, target_is_windows):

     if compiler.type == 'gcc':

+        #profile_use = '-fprofile-use=$(topobjdir)'

+        profile_use = '-fprofile-use'

         return namespace(

             gen_cflags=['-fprofile-generate'],

             gen_ldflags=['-fprofile-generate'],

-            use_cflags=['-fprofile-use', '-fprofile-correction',

+            use_cflags=[profile_use, '-fprofile-correction',

                         '-Wcoverage-mismatch'],

-            use_ldflags=['-fprofile-use'],

+            use_ldflags=[profile_use],

         )

     if compiler.type in ('clang-cl', 'clang'):

@@ -192,13 +194,13 @@ def lto(value, c_compiler, ld64_known_go

                 cflags.append("-flto")

                 ldflags.append("-flto")

             else:

-                cflags.append("-flto=thin")

-                ldflags.append("-flto=thin")

+                cflags.append("-flto")

+                ldflags.append("-flto")

         elif c_compiler.type == 'clang-cl':

             if len(value) and value[0].lower() == 'full':

                 cflags.append("-flto")

             else:

-                cflags.append("-flto=thin")

+                cflags.append("-flto")

             # With clang-cl, -flto can only be used with -c or -fuse-ld=lld.

             # AC_TRY_LINKs during configure don't have -c, so pass -fuse-ld=lld.

             cflags.append("-fuse-ld=lld");

diff -up a/build/pgo/profileserver.py.pgo b/build/pgo/profileserver.py

--- a/build/pgo/profileserver.py.pgo	2020-09-30 19:41:10.000000000 +0200

+++ b/build/pgo/profileserver.py	2020-10-01 13:54:58.535312905 +0200

@@ -11,7 +11,7 @@

 import subprocess

 import mozcrash

-from mozbuild.base import MozbuildObject, BinaryNotFoundException

+from mozbuild.base import MozbuildObject, BinaryNotFoundException, BuildEnvironmentNotFoundException

 from mozfile import TemporaryDirectory

 from mozhttpd import MozHttpd

 from mozprofile import IceCatProfile, Preferences

@@ -84,9 +84,22 @@ if __name__ == '__main__':

                        port=PORT,

                        options='primary,privileged')

-    old_profraw_files = glob.glob('*.profraw')

-    for f in old_profraw_files:

-        os.remove(f)

+    using_gcc = False

+    try:

+        if build.config_environment.substs.get('CC_TYPE') == 'gcc':

+            using_gcc = True

+    except BuildEnvironmentNotFoundException:

+        pass

+

+    if using_gcc:

+        for dirpath, _, filenames in os.walk('.'):

+            for f in filenames:

+                if f.endswith('.gcda'):

+                    os.remove(os.path.join(dirpath, f))

+    else:

+        old_profraw_files = glob.glob('*.profraw')

+        for f in old_profraw_files:

+            os.remove(f)

     with TemporaryDirectory() as profilePath:

         # TODO: refactor this into mozprofile

@@ -191,6 +204,11 @@

             print("IceCat exited successfully, but produced a crashreport")

             sys.exit(1)

+

+        print('Copying profile data....')

+        os.system('pwd');

+        os.system('tar cf profdata.tar.gz `find . -name "*.gcda"`; cd ..; tar xf instrumented/profdata.tar.gz;');

+

         llvm_profdata = env.get('LLVM_PROFDATA')

         if llvm_profdata:

             profraw_files = glob.glob('*.profraw')

diff -up a/build/unix/mozconfig.unix.pgo b/build/unix/mozconfig.unix

--- a/build/unix/mozconfig.unix.pgo	2020-09-30 19:41:10.000000000 +0200

+++ b/build/unix/mozconfig.unix	2020-10-01 13:54:58.535312905 +0200

@@ -6,6 +6,15 @@ if [ -n "$FORCE_GCC" ]; then

     CC="$MOZ_FETCHES_DIR/gcc/bin/gcc"

     CXX="$MOZ_FETCHES_DIR/gcc/bin/g++"

+    if [ -n "$MOZ_PGO" ]; then

+        if [ -z "$USE_ARTIFACT" ]; then

+            ac_add_options --enable-lto

+        fi

+        export AR="$topsrcdir/gcc/bin/gcc-ar"

+        export NM="$topsrcdir/gcc/bin/gcc-nm"

+        export RANLIB="$topsrcdir/gcc/bin/gcc-ranlib"

+    fi

+

     # We want to make sure we use binutils and other binaries in the tooltool

     # package.

     mk_add_options "export PATH=$MOZ_FETCHES_DIR/gcc/bin:$PATH"

diff -up a/extensions/spellcheck/src/moz.build.pgo b/extensions/spellcheck/src/moz.build

--- a/extensions/spellcheck/src/moz.build.pgo	2020-09-30 19:41:17.000000000 +0200

+++ b/extensions/spellcheck/src/moz.build	2020-10-01 13:54:58.535312905 +0200

@@ -31,3 +31,5 @@ EXPORTS.mozilla += [

 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):

     CXXFLAGS += ['-Wno-error=shadow']

+

+CXXFLAGS += ['-fno-devirtualize']

\ No newline at end of file

diff -up a/python/mozbuild/mozbuild/build_commands.py.pgo b/python/mozbuild/mozbuild/build_commands.py

--- a/python/mozbuild/mozbuild/build_commands.py.pgo	2020-09-30 19:41:46.000000000 +0200

+++ b/python/mozbuild/mozbuild/build_commands.py	2020-10-01 13:54:58.535312905 +0200

@@ -108,7 +108,8 @@ class Build(MachCommandBase):

                 return status

             pgo_env = os.environ.copy()

-            pgo_env['LLVM_PROFDATA'] = instr.config_environment.substs.get('LLVM_PROFDATA')

+            if instr.config_environment.substs.get('CC_TYPE') != 'gcc':

+                pgo_env['LLVM_PROFDATA'] = instr.config_environment.substs.get('LLVM_PROFDATA')

             pgo_env['JARLOG_FILE'] = mozpath.join(orig_topobjdir, 'jarlog/en-US.log')

             pgo_cmd = [

                 instr.virtualenv_manager.python_path,

--- a/toolkit/components/terminator/nsTerminator.cpp.pgo	2020-10-07 09:22:26.698243673 +0200

+++ b/toolkit/components/terminator/nsTerminator.cpp	2020-10-07 09:22:43.026312999 +0200

@@ -418,6 +418,11 @@ void nsTerminator::StartWatchdog() {

   }

 #endif

+  // Disable watchdog for PGO train builds - writting profile information at

+  // exit may take time and it is better to make build hang rather than

+  // silently produce poorly performing binary.

+  crashAfterMS = INT32_MAX;

+

   UniquePtr<Options> options(new Options());

   const PRIntervalTime ticksDuration = PR_MillisecondsToInterval(1000);

   options->crashAfterTicks = crashAfterMS / ticksDuration;